1. Field of the Invention
The present invention relates to an interferometer, and more particularly to a double pass interferometer which obtains displacement information of an object to be measured from lights which have been reflected twice on a reference mirror and a measurement mirror, respectively.
2. Description of the Related Art
Conventionally, as an apparatus which measures a displacement of an object, controls a stage, or performs a various kind of length measurements, a laser interferometer has been used because of the features of the high accuracy and high resolution. For example, Japanese Patent Laid-Open No. 2006-112974 discloses a position detecting apparatus which detects a position displacement of an object using an interference length measurement by non-contact.
FIG. 3 is a configuration diagram of a conventional interferometer. A laser beam 110 having a wavelength of λ (λ=633 nm) emitted from a light source 10 enters a PBS 20 (a polarizing beam splitter) and is split into reference light 120a and measurement light 120b on the PBS surface 20p. The reference light 120a is reflected on the reference mirror 40a and enters the PBS 20 again by passing through the previous optical path. In this case, A P wave is converted to an S wave by being transmitted through a ¼ λ plate 30a twice. Therefore, it is transmitted through the PBS surface 20p to be reference light 130a and enter a reflective device 50.
On the other hand, the measurement light 120b is reflected on a measurement mirror 40b and passes through a previous optical path to enter the PBS 20 again. In this case, because a light beam of the measurement light 120b is transmitted through a ¼ λ plate 30b twice and an S wave is converted to a P wave, it is reflected on the PBS surface 20p to be a light beam 130b and, similarly to the reference light 130a, enter the reflective device 50.
After that, the reference light 130a is transmitted through the PBS 20 again to be reference light 140a, and the measurement light 130b is reflected on the PBS 20 again to be measurement light 140b. The reference light 140a and the measurement light 140b are transmitted through the ¼ λ plates 30a and 30b twice, respectively. The reference light 140a and the measurement light 140b entered the PBS 20 again are multiplexed to be multiplexed light 150. An interference signal having a period of ¼ λ in accordance with a displacement of the measurement mirror 40b can be obtained by receiving the multiplexed light 150 by a light receiving device 160.
As shown in FIG. 3, in a conventional typical interferometer, there are a lot of reflective surfaces in the optical path. A component reflected on an interface 210b passes through the same optical path as that of the ordinary measurement light and is finally superimposed on the multiplexed light 150. Although this reflective component is modulated by the movement of the measurement mirror 40b, it reaches the measurement mirror 40b by only one reflection. The same is true for interfaces 210a, 210aa, and 210bb. Therefore, a modulation amount is a half of an ordinary reflective component and is obtained as an interference signal (a ghost light signal) having a period of ½ λ.
For example, when a reflectance of an AR coat (an antireflective coating film) on the interface 210b is 0.2%, the interference signal generated by the ghost light which has been reflected on the interface 210b has an interference intensity of no less than around 9%, compared wave-optically to the interference intensity of a primary signal. Even if an ultralow reflective AR coat having a reflectance of 0.01% is adopted, the interference intensity is 2.5%.
FIG. 4A is a waveform of an ideal interference signal, and FIG. 4B is a waveform of an interference signal on which a ½ λ periodic error is superimposed. The interference signal shown in FIG. 4B is a periodic signal including an error caused by ghost light of a double pass interferometer.
On the electric signal outputted from the light receiving device 160, a sine wave signal caused by all ghost lights is superimposed. Therefore, the interference signal obtained by the conventional interferometer has a waveform as shown in FIG. 4B.
FIG. 5 is a diagram showing a relationship between an output of a light receiving device (a sensor displacement output) and a displacement of an object to be measured.
Commonly, sub-nanometer resolution can be obtained by electrically dividing the sine wave periodic signal modulated in accordance with the displacement of the object to be measured. However, if a component caused by the ghost light is superimposed, as shown in FIG. 5, the linearity of the sensor displacement output with respect to the displacement of the object to be measured is deteriorated. In other words, an interpolation error is included between the sensor displacement output and the displacement of the object to be measured. Because this error amount reaches several nanometers to tens of nanometers, it is a big problem in the interferometer used for the application requiring ultrahigh accuracy